Comparison of the Size of Perivitelline Space in Mouse Oocytes Cultured with and without Cumulus Cells and Possible Hyaluronan Production in Oocytes

Perivitelline space is the gap between the plasma membrane and zona pellucida of oocytes. Recently, it has been reported that the incidence of polyspermy in porcine and mouse oocytes with larger perivitelline space is significantly lower than those with small perivitelline space (Funahashi et al., 1994; Wang et al., 1998; Kitagawa and Niimura, 2006; Ueno and Niimura, 2008). Therefore, it has been suggested that there could be a relationship between the size of perivitelline space and the incidence of polyspermy in porcine and mouse oocytes. It is known that glycosaminoglycans including hyaluronan synthesized in cumulus cells by the stimulation of FSH and LH are secreted (Eppig, 1982; Ball et al., 1985) and the matrix of cumulus cells is occupied by the secreted hyaluronan (Eppig, 1979; 1980; Ball et al., 1982; Salustri et al., 1992). Hyaluronan has been found to also exist in the perivitelline space (Talbot and Dicarlantonio, 1984a; Dandekar and Talbot, 1992; Dandekar et al., 1992; Talbot and Dandekar, 2003), and one of its properties is retention of large volumes of water (Talbot and Dandekar, 2003). Therefore, when hyaluronan accumulates in the perivitelline space, its role is to absorb water causing the perivitelline space to enlarge. On the other hand, with regard to the secretory source of hyaluronan in the perivitelline space, it has been confirmed by electron microscopy that hyaluronan exists in the pore of the zona pellucida (Talbot and Dicarlantonio, 1984b). From these results, it was suggested that hyaluronan secreted from cumulus cells enters the perivitelline space through the zona pellucida (Talbot,1984). However, when the stimulation of FSH and LH occurs, glycosaminoglycans including hyaluronan are secreted not only from cumulus cells but also from oocytes (Pivko et al., 1982; Tesarik and Kopecny, 1986; Fléchon et al., 2003). In addition the mRNA of HAS3, which is a hyaluronan synthase, is present in porcine oocytes (Kimura et al., 2002). Moreover, when ultra thin sections of hamster cumulus-oocyte complex were stained with immunogoldlabeled hyaluronidase, deposition of gold particles was observed in cumulus cells, perivitelline space and the matrix